The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435;42 U.S.C.2457).
The invention relates to an apparatus and method of operation thereof for heat treating a disk so as to produce a dual microstructure superalloy disk particularly suited for gas turbine applications.
There are numerous incidents where operating conditions experienced by an article, or a component of a machine, place different material property requirements on different portions of the article or component. Examples include a crank shaft in an internal combustion engine, a piston rod in a hydraulic cylinder, planatary gears for an automotive transmission, and a turbine disk for a gas turbine engine. Gas turbine disks are often made from nickel-base superalloys, because these disks need to withstand the temperature and stresses involved in the gas turbine cycle. In the bore portion of the disk where the operating temperature is somewhat lower, the limiting material properties are often tensile strength and low-cycle fatigue resistance. In the rim portion of the disk, where the operating temperatures are higher than those of the bore, because of the proximity to the combustion gases, resistance to creep and cracking are the limiting properties.
Advanced nickel-base, gamma prime strengthened superalloys have been introduced to the field that allow improved engine performance through higher disk temperatures as compared to current engines. This is achieved by using high levels of gamma prime and refractory elements. However, there is a long term need for disks with higher rim temperature capabilities of 1400xc2x0 F. or more. This increased temperature capability would allow higher compressor exit temperatures of a gas turbine and allow the full utilization of advanced combustion and airfoil concepts for aerodynamic applications. These disks require high creep resistance and dwell crack growth resistance of coarse grain microstructures in the rim region near 1400xc2x0 F., while still maintaining the high strength and low cycle fatigue resistance of fine grain microstructures in the bore region near 800-1200xc2x0 F.
The chief determinant of achieving grain size in powder metallurgy superalloy disks is the temperature at which the alloy is solution heat treated. As is known in the art, solution heat treatment is concerned with the solvus temperature; i.e., the temperature at which all of the gamma prime strengthening precipitate of the superalloy goes into solution. To perform the desired solution heat treatment in this invention, it is necessary to solution heat treat the disk in a way whereby the rim is heated to a higher solution heat treatment temperature than the bore. Furthermore, it would be necessary at the same time, as known in the art, to be able to directly quench the disk after the solution heat treatment to achieve high tensile strength and low cycle fatigue resistance in the bore and high creep resistance in the rim.
For most gas turbine applications, disks are currently heat treated at uniform solution temperature either below the gamma prime solvus temperature (subsolvus heat treatments), or above the solvus temperature (supersolvus heat treatments). Several recent approaches have been established which differ from the traditional subsolvus or supersolvus heat treatment. One approach, more fully described in U.S. Pat. No. 5,312,497, uses induction heating to preferentially heat the rim of a disk, while a pressurized gas is run through the bore of the disk to keep the bore and web cooler. Another approach, more fully described in U.S. Pat. Nos. 5,527,020 and 5,527,402, uses simpler top and bottom thermal caps placed over the bore of the disk to blow pressurized air through the center of a single disk, while the disk is being held at a constant temperature in a gas fired furnace. In this way, the bore of the disk is maintained at a sufficiently cooler temperature than the rim of the disk, thus, achieving desired subsolvus solution of the bore and desired supersolvus solution of the rim.
Uniform disk temperature heat treatments produce either fine or coarse grain microstructures throughout the disk. The fine grain microstructure has inferior creep and dwell crack growth resistance for rim service temperatures. Similarly, the coarse grain microstructure has inferior tensile and low cycle fatigue resistance for bore service temperatures. The approach described in U.S. Pat. No. 5,312,497, using induction heating of the rim with pressurized gas cooling of the bore can only be applied to one disk at a time, and is thereby very expensive. The practice of U.S. Pat. No. 5,312,497 is also very sensitive to induction coil-disk geometry tuning, disadvantageously yielding difficult process control. The approach described in U.S. Pat. Nos. 5,527,020 and 5,527,402, also is limited to heat treating one disk at a time. The practice of U.S. Pat. Nos. 5,527,020 and 5,527,402, while having reduced complexity compared to the practice of U.S. Pat. No. 5,312,497, still requires specialized air pressure lines going into a furnace that must remain operable for process viability. Accordingly, there still remains a need to provide heat treatment devices, and methods of use thereof, that provide different microstructures in the bore and rim portions of nickel-base superalloy disks without suffering the drawbacks of the prior art techniques.
It is a primary object of the present invention to provide a heat treatment apparatus and method of use thereof. The heat treatment yields rim portions of superalloy disks as having higher temperature capabilities associated with coarse grain microstructures, while at the same time maintaining high strength and low cycle fatigue resistance of fine grain microstructures in the bore portions of superalloy disks near 800-1200xc2x0 F.
It is another object of the present invention to provide for different microstructures in the bore and rim portions of nickel-base superalloy disks and accomplish such by the use of standard production furnaces without auxiliary cooling.
It is further desired to provide differential microstructures in the rim and bore portions of nickel-base superalloy disks while still maintaining the option for rapid cooling upon completion of the solution heat treatment using conventional fan or oil quenching operations.
A further object of the present invention is to provide for design of the heat treatment device using a finite element computer code and solvus data of the disk alloy.
This invention is directed to a heat treatment apparatus and methods of use thereof which produce different microstructures in the bore and rim portions of nickel-base superalloy disks particularly suited for gas turbine engines.
In one embodiment, an apparatus is provided that is insertable and removable from a heat treatment furnace for differentially heat treating a superalloy disk to obtain a dual microstructure disk. The disk comprises an inner section termed the bore with a bore hole, an intermediate section termed the web portion, an outer section termed the rim portion, and first and second faces on opposite sides of the disk. The disk has predetermined diameter and thickness dimensions. The apparatus comprises first and second thermal blocks, respectively, arranged on the first and second faces of the disk. Each of the first and second thermal blocks has predetermined diameter and thickness dimensions related to the predetermined diameter and thickness dimensions of the disk by a predetermined relationship. The diameters of the first and second thermal blocks are less than the diameter of the disk. The first and second thermal blocks each have have upper and lower faces with the lower face of the first thermal block having an alignment pin positionable in correspondence with the bore hole of the disk and the upper face of the second thermal block having an alignment pin positionable in correspondence with the bore hole of the disk so that the first and second thermal blocks along with the disk are brought together and expose at least the rim portion of the disk. The apparatus further comprises first and second insulation jackets that surround the first and second thermal blocks. Each insulating jacket consists of an alignment plate, outer shell, and insulating media. The first and second alignment plates are respectively fastened to the upper face of the first thermal block and to the lower face of the second thermal block. The alignment plates have diameters greater than the thermal blocks. The apparatus still further comprises first and second outer shells respectively located outside of the first and second alignment plates with high temperature insulating media filling the cavity between the outer shells and thermal blocks.
The invention provides a method for differentially heat treating a superalloy disk having a gamma prime solvus temperature so as to obtain a dual microstructure disk. The method includes providing first and second thermal blocks respectively arranged on first and second faces of a disk. Each of the first and second thermal blocks has predetermined diameter and thickness dimensions related to the predetermined diameter and thickness dimensions of the disk by a predetermined relationship. The first and second thermal blocks each has upper and lower faces, with the lower face of the first thermal block having an alignment pin positionable in correspondence with the bore hole of the disk, and the upper face of the second thermal block having an alignment pin positionable in correspondence with the bore hole of the disk. The diameters of the first and second thermal blocks are less than the diameter of the disk. The method further includes providing first and second alignment plates each with a diameter greater than the diameter of the first and second thermal blocks and having means for being respectively fastened to the upper face of the first thermal block and to the lower face of the second thermal block. The method further comprises providing first and second outer shells respectively located outside of the first and second alignment plates with high temperature insulating media filling the cavity between the thermal blocks and outer shells. The method further includes the following steps: (1) positioning each of the alignment pins of the first and second thermal blocks in correspondence with the bore hole of the disk; (2) bringing together the first and second thermal block, the first and second shells with the associated high temperature insulating media and the disk thereby exposing the rim portion of the disk; (3) selectively attaching a thermocouple to either the first or second thermal block; (4) placing the brought together disk, the first and second thermal blocks, the first and second shells with the associated high temperature insulating media, and the thermocouple in a furnace; (5) heat treating the disk with heat sink assembly in a standard production furnace maintained at a temperature which is above the gamma prime solvus temperature of the disk for a first predetermined duration; (6) removing the disk and heat sink assembly from the furnace when the thermocouple reaches the subsolvus temperature of the disk alloy; (7) freeing the disk from the heat sink assembly; and (8) quenching the disk.